1. Field of the Invention
The present invention relates to optics. More particularly, this invention relates to athermalized and orthoscopic lens systems. Still more particularly the invention relates to a wide angle lens system, color and distortion corrected over the visible spectrum and athermalized over a wide (e.g. −60° C. to 80° C.) temperature range.
This lens is suitable for, but not limited to, visible and near infrared airborne sensor systems where athermalized, achromatized and orthoscopic performance is essential for high resolution target imaging.
2. Brief Description of Related Art
U.S. Pat. No. 5,691,847 discloses an athermalized and achromatized optical system employing a diffractive optical element. This lens includes a refractive lens group and diffractive optical element. The refractive group consists of elements of at least two different materials for thermal compensation and a diffractive optical element which is used for chromatic aberration correction. This lens is used in the infrared spectrum and the claimed relations between the lens parameters can not be utilized in the visible spectrum. This lens is also not orthoscopic and has a modest field of view. The diffractive element also introduces stray light which may not be acceptable in some applications.
Another example of a wide field lens is disclosed in U.S. Pat. No. 6,023,375. This projection lens is used to form an image of a pixelized panel (for example LCD) on a screen and possesses the following properties: long focal length, magnification adjustment, relatively small size and relatively low distortion (˜1% or about 100 microns). The lens consists of components with positive and negative optical powers with certain lens elements having weak or strong optical powers. In order to address the temperature sensitivity problem and weight issues the lens utilizes both glass and plastic materials. For distortion and field curvature correction a negative optical power field lens is used. The lens is not completely athermal through a large temperature range and residual distortion is large enough to prevent usage of this lens as a high performance orthoscopic lens system where distortion is required to be less than 0.04%.
Another athermalized lens is disclosed in U.S. Pat. No. 6,999,243. This lens combines refractive and diffractive optical surfaces with special relations between elements optical powers; also materials which are transmissive in IR spectrum are utilized. This configuration is not suitable for the lenses working in visible spectrum, the field of view is relatively small and diffractive surfaces may introduce scattered light glare which reduces contrast at the image plane.
Yet another wide field of view projection lens is described in U.S. Pat. No. RE 39,911. This lens has a wide field of view of at least 35°, utilizes an aspherical surface at the first element for distortion correction and spherical surfaces for aberrational correction. However this lens is not free from the performance drawbacks described above. The residual distortion is nearly 1% and spherochromatism exceeds 10 microns.
Yet another imaging wide field of view lens is described in U.S. Pat. No. 7,405,890. This lens operates over the visible and near infrared spectrums, is compact and provides a wide field of view. However this lens is not athermalized and is not orthoscopic. Residual distortion exceeds 2-3% and spherochromatism is 20-30 microns.